Global System for Mobile Communications (GSM) is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by mobile phones. The GSM standard originally described a digital, circuit-switched network optimized for full duplex voice telephony, and was expanded over time to include data communications, first by circuit-switched transport, then packet data transport via General Packet Radio Services (GPRS) and Enhanced Data rates for GSM Evolution (EDGE). Further improvements were made when the 3rd Generation Partnership Project (3GPP) developed third generation (3G) Universal Mobile Telecommunication System (UMTS) standards followed by fourth generation (4G) Long Term Evolution (LTE) Advanced standards.
In the radio access network of a wireless communication system such as GSM, UMTS or LTE, a wireless device is wirelessly connected to a Radio Base Station (RBS). An RBS is a general term for a radio network node capable of transmitting radio signals to a wireless device and receiving signals transmitted by the wireless device. The Base Transceiver Station (BTS) is the RBS in GSM. In GSM, another radio network node—a so called Base Station Controller (BSC)—provides the intelligence behind the BTS. Typically a BSC has several BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, and controls intra-BSC handovers from BTS to BTS. In UMTS and LTE, the RBS is commonly referred to as a NodeB and an evolved NodeB (eNodeB) respectively. In UMTS, a Radio Network Controller (RNC) controls a number of NodeBs, and is in charge of management of radio resources in cells for which the RNC is responsible. In LTE, the eNodeB is the only radio network node. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.
FIG. 1a illustrates a radio access network in a UMTS or a GSM, with an RBS 20 wirelessly connected to a wireless device 40 located within the RBS's geographical area of service, called a cell 30. An RNC 10 in UMTS or a BSC 10 in GSM controls the RBS 20. The RNC/BSC 10 serves the wireless device 40 in the cell 30 via the RBS 20. The RNC/BSC 10 is connected to the Core Network (CN) (not illustrated). FIG. 1b illustrates a radio access network in an LTE system. An eNodeB 50 serves a User Equipment (UE) 40 located within the cell 30. The eNodeB 50 is directly connected to the CN (not illustrated).
One of the most challenging scenarios in a broadband wireless access technology is a high mobility scenario, such as scenarios in the high speed railway domain. A high speed railway introduces quite specific challenges especially with regards to the handover and cell change procedure. When a wireless device is moving very fast as when it is in a High Speed Train (HST), the wireless device has a limited amount of time to measure neighboring cells before a handover.
In existing solutions, a separate HST network, i.e., a wireless network comprising cells covering only the high speed railway, is created. The HST network cells 200 are overlapping the normal cellular network 210, as illustrated in FIG. 2a. The HST network cells 200 are deployed as a trail of cells along the railroad track. With a separate HST network, the network and handover procedures can be optimized for the high speed of the wireless devices.
However, the separation of HST networks and normal cellular networks does not work optimally as wireless devices which are not travelling on HSTs ends up using the HST network anyhow. The penetration loss of a HST is often high as the railroad cars have a wholly-enclosed structure. Therefore, the signal strength used in RBSs of the separate HST needs to be high in order to penetrate the railroad cars. This makes it hard to exclude non-HST devices from the HST network, as the devices want to access or camp on the cell with highest signal strength. Non-HST devices may thus steal capacity from devices travelling on HST trains. This results in insufficient capacity for devices travelling on HST trains. HST devices that could not be accommodated by the HST network may end up in the normal network with a high probability for dropped calls for a HST-device. Furthermore, the solution providing mobility between the networks is complex compared to mobility in an integrated network. Considering that the traffic load of a separate HST network in general is low, such a solution becomes costly.
Other existing solutions are based on normal cellular networks where the cell deployment is adapted to provide support for HST scenarios. FIG. 2b illustrates a normal cellular network where some cells 230 are defined as HST cells and configured to support the HST scenario, thus allowing for accurate handover procedures for high speed devices. One example may be to increase the cell overlapping areas between these cells 230. Such a solution is not flexible, as the cells are always adapted for HST scenarios, although HST only passes the cells occasionally. An alternative solution comprises performing speed estimations of wireless devices in order to adapt the mobility parameters for a wireless device that moves at high speed. However, such a solution is dependent on accurate positioning methods for the device, and speed estimations consume extra power and signaling resources.